In sweeping proposals released after four years of frustrated efforts, Mr Obama ordered new curbs on carbon emissions from power plants and called for America to ready its defences against an already-changing climate.

The president also surprised environmentalists by signalling he would reject a controversial oil pipeline if it was found to “significantly exacerbate” the problem of carbon pollution.

“I refuse to condemn your generation and future generations to a planet that is beyond fixing,” Mr Obama told students at Georgetown University. “As a president, as a father and as an American I’m here to say we need to act.”

Mr Obama first promised a new push on climate change during his inagural address on a freezing morning in January, warning that failing to cut emissions “would betray our children”.

Today, as Washington sweltered in the summer heat, Mr Obama finally laid out the details of his plan, which can be carried out by executive order and therefore do not need Congressional approval.